Singular/Plural Rules and Examples from Scientific Writing

In English, a noun is indicated as singular or plural by its form, by its demonstrative (e.g., those, these) and by the verb. By default, words are pluralized by adding -s, but there are multiple exceptions in academic writing.

The regular plural: -s and -es

Singular ending	Plural	Examples
Most nouns	add -s	sample → samples; result → results; gene → genes; peak → peaks
-s, -ss, -sh, -ch, -x, -z	add -es	gas → gases; class → classes; dish → dishes; batch → batches; box → boxes; quiz → quizes
Consonant + -y	-y → -ies	study → studies; frequency → frequencies; artery → arteries; anomaly → anomalies
Vowel + -y	add -s	array → arrays; survey → surveys; key → keys
-o	varies	potato → potatoes; tomato → tomatoes; echo → echoes; hero → heroes but photo → photos; ratio → ratios; zero → zeros; kilo → kilos; video → videos; embryo → embryos
-f / -fe	often -ves	leaf → leaves; half → halves; knife → knives; shelf → shelves; wolf → wolves but roof → roofs; belief → beliefs; chief → chiefs; proof → proofs; safe → safes

Keep in mind the following:

• -o nouns: the -es forms are mostly old, fully naturalized words. Technical and abbreviated words (photo, kilo, ratio, silo, proto) take plain -s. A few allow both: volcano(e)s, mosquito(e)s, tornado(e)s, zero(e)s.
• -f nouns: a handful take either form — scarf → scarves/scarfs, hoof → hooves/hoofs, dwarf → dwarves/dwarfs.

Irregular plurals

Some English nouns change their vowels or take an alternative ending instead of -s. These must be memorized, but luckily there are comparatively few of these.

Type	Examples
Vowel change (mutation)	man → men; woman → women; foot → feet; tooth → teeth; goose → geese; mouse → mice; louse → lice
-en plural	child → children; ox → oxen
Zero plural (same form both ways)	sheep, deer, fish, series, species, aircraft, offspring, salmon, trout, means

Fish and fishes are both correct but differ in sense: fish counts individuals, fishes counts species or kinds. The same distinction applies to peoples (ethnic groups) versus people (persons).

Countable vs uncountable nouns

Countable nouns can be numbered and have both forms. Uncountable (mass) nouns cannot be pluralized and cannot take a/an.

	Countable	Uncountable
Article	a sample, an experiment	equipment, research
Plural	samples, experiments	equipment, research
Quantifiers	many, few, several, a number of	much, little, a great deal of, a large amount of
Counting phrase	three samples	three pieces of equipment

Common uncountable nouns in academic writing

These take no plural and no indefinite article. Substituting a plural is one of the most frequent errors in manuscripts by non-native speakers.

Uncountable noun	Right
research	recent research shows; recent studies show
evidence	several lines of evidence; considerable evidence
equipment	three pieces of equipment; three instruments
information	information was collected
literature	the literature suggests
software	two software packages
knowledge	our knowledge
feedback, personnel, apparatus, advice, progress	treat as mass nouns

Plurals of compound nouns

Some compound nouns are pluralized in specific ways:

Compound	Plural	Main noun
mother-in-law	mothers-in-law	mother
editor-in-chief	editors-in-chief	editor
passer-by	passers-by	passer
attorney general	attorneys general	attorney
cul-de-sac	culs-de-sac	cul
cross-section	cross-sections	section (final)
follow-up	follow-ups	up (final)
spoonful	spoonfuls	Final

When the compound is written as one word, the plural simply goes on the end: databases, subsets, timepoints, wavelengths.

Plural patterns in academic English

Most of the irregular plurals in scientific English follow one of six patterns inherited from Latin or Greek.

Ending	Plural	Examples (singular → plural)
-um	-a	bacterium → bacteria; datum → data; medium → media; ovum → ova; septum → septa; stratum → strata; flagellum → flagella; cilium → cilia; epithelium → epithelia; inoculum → inocula; erratum → errata
-us	-i	nucleus → nuclei; locus → loci; fungus → fungi; bacillus → bacilli; coccus → cocci; stimulus → stimuli; villus → villi; thrombus → thrombi; radius → radii; alumnus → alumni
-a	-ae	larva → larvae; alga → algae; vertebra → vertebrae; pupa → pupae; hypha → hyphae; lamina → laminae; ulna → ulnae
-on	-a	criterion → criteria; phenomenon → phenomena; ganglion → ganglia; mitochondrion → mitochondria; protozoon → protozoa
-is	-es	analysis → analyses; hypothesis → hypotheses; basis → bases; axis → axes; diagnosis → diagnoses; prognosis → prognoses; thesis → theses; synthesis → syntheses; metastasis → metastases; anastomosis → anastomoses
-ex / -ix	-ices	matrix → matrices; vertex → vertices; vortex → vortices; cortex → cortices; apex → apices; helix → helices; calyx → calyces

A smaller Greek group takes -x → -ges or -ma → -mata:

• phalanx → phalanges; larynx → larynges; meninx → meninges; thorax → thoraces
• stoma → stomata; carcinoma → carcinomata (though carcinomas is now standard in clinical writing)

Examples:

• ✗ “A single bacteria was recovered from each plate.” → ✓ “A single bacterium was recovered.”
• ✗ “The main criteria for inclusion was age over 60.” → ✓ “The main criterion for inclusion was age over 60.”
• ✗ “This phenomena has been reported previously.” → ✓ “This phenomenon has been reported previously.”
• ✗ “Cells were grown in a liquid media.” → ✓ “Cells were grown in a liquid medium.”

 

Words with two acceptable plurals

Some borrowed words have developed an anglicized plural alongside the classical one. In several cases the two forms have split by meaning, and using the wrong one reads as careless.

Singular	Classical plural	English plural	Convention
appendix	appendices	appendixes	Appendices for supplementary document sections; appendixes for the anatomical organ
index	indices	indexes	Indices for mathematical subscripts, exponents, and economic measures; indexes for alphabetical lists in books
formula	formulae	formulas	Formulae is common in chemistry and mathematics; formulas is acceptable everywhere and now more usual
antenna	antennae	antennas	Antennae on insects; antennas on radios
cannula	cannulae	cannulas	Both used; follow your target journal’s style
focus	foci	focuses	Foci for lesions, optical points, and outbreaks; focuses mainly as a verb form
curriculum	curricula	curriculums	Curricula strongly preferred
symposium	symposia	symposiums	Symposia preferred
memorandum	memoranda	memorandums	Memoranda preferred in formal writing

Once you choose a form, use it consistently throughout the manuscript.

 

False Latin plurals

Not every word ending in -us is a Latin noun. Several common science words take regular -es plurals, and “correcting” them produces a well-known category of error.

Word	Correct plural
virus	Viruses
apparatus	apparatus or apparatuses
prospectus	prospectuses
status	statuses
syllabus	syllabuses or syllabi
octopus	octopuses
campus	campuses
bonus	bonuses
ignoramus	ignoramuses

 

Words that are commonly confused

Same in singular and plural

Number is signalled only by the article, demonstrative (this vs these), or verb:

• Species: “This species is endemic to Borneo.” / “These species are morphologically similar.”
• Series: “A series of dilutions was prepared.” / “Three series were run in parallel.”
• apparatus, means, forceps, fish (though fishes denotes multiple species), sheep, deer, offspring, aircraft

Singular words that look plural

Names of disciplines ending in -ics:

• “Statistics is a required course for all graduate students.” (the field)
• “These statistics are drawn from the national registry.” (the numbers)
• Similarly: mathematics, physics, genetics, kinetics, dynamics, mechanics, optics, economics, informatics
• Also singular: news, mumps, measles, rickets

Academic collective nouns

Collective nouns such as cohort, group, team, committee, majority, population:

• American usage treats them as singular: “The cohort was followed for five years.”
• British usage allows plural when members act individually: “The committee were divided in their assessment.”

Pick one convention and stay with it.

 

 

Units and measurements

Here are the main rules around pluralization of numbers and units.

Rule	Example
Unit symbols never take -s	5 kg, 10 mL, 25 °C, 3 h, 40 mg — never , ,
Unit names spelled out do pluralize	five kilograms, ten milliliters, three hours
Exception: lux, hertz, siemens, lumen symbols and some names stay fixed	50 lux, 60 Hz
A space separates number and symbol (except % and °)	10 mg, 37°C, 95%

 

Abbreviations, symbols, and numerals

• Form plurals of abbreviations by adding -s with no apostrophe: ELISAs, PCRs, ROIs, SNPs, CTs, PhDs, IQs.
• Decades and number groups: “the 1990s,” “three CD4s,” “values in the 20s.”
• Italic statistical symbols pluralize with a roman s: ns, ps, rs. Many journals prefer rephrasing (“the sample sizes”) to avoid the awkward form.
• et al. always refers to multiple authors and takes a plural verb: “Nakamura et al. report a similar effect.” (The period after al. is required; et takes none.)
• sp. is one unnamed species; spp. is several: “Pseudomonas sp. was isolated” / “several Candida spp. were detected.”
• e.g. and i.e. are unchanged regardless of how many items follow.

 

Hyphenated modifiers

When a measurement modifies a noun, it becomes singular and hyphenated:

• a 5-year follow-up (not )
• a 20-mg dose, two 20-mg doses
• a 10-fold increase
• a three-week incubation
• a 96-well plate
• a two-step protocol

The same applies to other attributive nouns: a sample collection protocol, patient records, cell viability, dose–response curve. The exceptions are where the plural is fixed by convention: systems biology, materials science, sports medicine, arms race.

 

Taxonomy and nomenclature

• Genus names are singular and capitalized and italicized; species epithets are lowercase: Escherichia coli. The genus plural is genera; the rank taxon pluralizes to taxa.
• A species name is grammatically singular even when the organisms are many: “E. coli was grown overnight.” To refer to countable organisms, add a noun: “E. coli cells were grown overnight.”
• Strain designations have a noun after them for plurals: “three S. aureus isolates“
• Gene symbols follow species-specific conventions and do not pluralize; write “the TP53 and BRCA1 genes.”

The “data” question

Data is the plural of datum, and most scientific style guides still enforce that:

• Plural by rule: APA, AMA, CSE, Nature, and most society journals use wording like “The data were analyzed using R.” “These data suggest a dose-dependent effect.”
• Singular mass noun: increasingly accepted in computing, business, and some general-interest publications, e.g., “The data is stored on a secure server.”

Datum is used mainly in surveying and geodesy, where it means a reference point. In most manuscripts, use data point, value, or observation for the singular rather than datum, and keep the plural verb unless the target journal says otherwise.

There’s also debate around the use of media (plural of medium), which stays firmly plural in laboratory contexts, and agenda, which is now considered singular (agendas is now the normal plural).

Possessives vs plurals

The apostrophe signals possession, never number. This is the single most common punctuation error in English writing.

Form	Meaning	Example
samples	plural	The samples were frozen.
sample’s	singular possessive	The sample’s pH was 7.2.
samples’	plural possessive	The samples’ pH values ranged from 6.8 to 7.4.
(as plural)	error

For irregular plurals not ending in -s, add ‘s: children’s, men’s, mice’s. For singular nouns already ending in -s, both Jones’s and Jones’ are accepted so check recent issues from your target journal and follow that.